1. Field of the Invention
The present invention generally relates to interface circuits between a transmission line (for example, a telephone line) and a modem adapted to transmitting and receiving data. Such interface systems especially have the function of isolating the transmission line from the rest of the user equipment, in particular due to voltage level differences between the user equipment and the signals carried by the transmission line. The present invention more specifically relates to interface systems using a barrier of capacitive isolation of the transmitted signals.
2. Discussion of the Related Art
The principle of such a capacitive isolation is based on a transposition of the useful frequency band (baseband) to a much higher frequency band by means of a modulation-demodulation system. The useful band ranges, in the example of telephone lines, from 300 to 3400 Hz. The transposition of the useful frequency band, necessary to transpose the telephone frequency band from 300 to 3400 Hz generally used as a carrier of data transmissions on the line, also enables decreasing the respective sizes of the capacitors to be used for the isolation barrier.
It is accordingly necessary to have, on either side of the capacitive isolation barrier through which the modulated signals transit, modulation and demodulation means that provide, either to the line or the user equipment, the signals in the useful frequency band.
FIGS. 1, 2, and 3 very schematically show a conventional example of interface with a capacitive isolation barrier between a transmission line and a modem. FIG. 1 shows in more detail the system portion on the modem side. FIG. 2 very schematically shows an example of a modulator for transposing the modem passband to the high isolation barrier crossing frequency (for example, a 1-MHz carrier). FIG. 3 shows in more detail the circuit on the line side.
An interface system to which the present invention relates, illustrated in FIGS. 1 to 3, is based on the use of an isolation barrier 1 formed with an assembly of capacitors C1, C2, C3, C4, C5, and C6 via which transit signals modulated on a high frequency carrier (for example, on the order of one MHz), between a processing circuit 2 on the equipment side (modem) and a processing circuit 3 on the line side.
As illustrated in FIG. 1, signal processing circuit 2 on the modem side essentially includes a modulator (MOD) 21 of the signals to be transmitted on the line and a demodulator (DEMOD) 22 of the signals received from the line. Modulator 21 receives the signals to be transmitted, in differential form, from two outputs Tx+ and Tx− of an amplifier (Tx) 23 having its differential inputs E+ and E− receiving the signals to be transmitted in the baseband (for example, from 300 to 3400 Hz). Amplifier 23 forms, for example, in a simplified way, the radiofrequency transmission head of the modem. Amplifier 23 is, for example, based on a low-noise amplifier.
Modulator 21 also receives a clock signal CK provided by a generator (GEN) 24. Clock signal CK corresponds, on the side of circuit 2, to the high-frequency carrier to which the transmission band must be transposed to pass isolation barrier 1. Two differential outputs St+ and St− of modulator 21 are respectively connected to a first armature of capacitors C1 and C2, the second respective armatures of which are connected to inputs Et+ and Et− of a demodulator (DEMOD) 31 of processing circuit 3 on the line side, as will be seen hereafter in relation with FIG. 3. To enable a proper crossing of isolation barrier 1, the modulation is performed with a carrier suppression, which amounts to multiplying the signal to be modulated by 1 or −1, at the frequency of clock CK.
Symmetrically, demodulator 22 of processing circuit 2 on the equipment side includes two differential inputs Er+ and Er− originating from respective first armatures of capacitors C3 and C4 of isolation barrier 1. The second respective armatures of capacitors C3 and C4 are connected, on the side of circuit 3, to two output terminals Sr+ and Sr− of a reception modulator (MOD) 32 that will be described hereafter in relation with FIG. 3.
Demodulator 22 is intended for restoring the signals received from the transmission line that have been modulated on the high-frequency carrier of crossing of isolation barrier 1, to provide these signals Rx+ and Rx− in differential form to a receive amplifier 25 (Rx). Amplifier 25 represents, for example, the receive head of the modem and provides, on two outputs S+ and S−, the signals received in differential form. Demodulator 22 also receives clock signal CK from generator 24 to enable the demodulation.
To enable recovering the data in the telephone band, be it on the user equipment side or on the line side, the modulation and demodulation clocks must be synchronous on either side of isolation barrier 1. For this purpose, the frequency of clock CK provided by circuit 24 is transmitted, from circuit 2 to circuit 3, in the form of two differential input signals CK+ and CK− transiting through two capacitors C5 and C6 of barrier 1.
FIG. 2 very schematically illustrates an example of structure of modulator 21 of the signals to be transmitted. Such a modulator is based on the use of switches K1, K2, K3, and K4 that are controlled by clock signal CK. Switches K1 and K2 receive, on a first terminal, signal Tx+. A second terminal of switch K1 forms output terminal St+ of the modulator while a second terminal of switch K2 is connected to output terminal St− of the modulator. Signal Tx− is sent onto the first respective terminals of switches K3 and K4. The second terminal of switch K3 is connected to terminal St+ while the second terminal of switch K4 is connected to terminal St−. Switches K1 and K2 are controlled to the on-state by signal CK while switches K3 and K4 are controlled to the off-state by signal CK. In other words, switches K1 and K2 are controlled by signal CK while switches K3 and K4 are controlled by the inverse of signal CK. In FIG. 2, these controls have been schematized by the direction of the arrows associated with the control terminals of switches K1, K2, K3, and K4. The operation of a multiplier such as illustrated in FIG. 2 is conventional and will not be detailed further.
As illustrated in FIG. 3, processing circuit 3 on the line side includes modulator 31 for restoring, in the transmission band, the signals to be transmitted that it receives in modulated form on terminals Et+ and Et−. The outputs of demodulator 31 are sent onto a transmission amplifier 33 (Tx) that provides the signals to be transmitted, transposed back into the base or useful band.
Demodulator 31 receives a clock signal CK′ from a circuit 34 (REGEN) for regenerating a clock signal synchronous with signal CK on the equipment side, based on a reprocessing of signals CK+ and CK− received as an input by circuit 34.
On the receive side, modulator 32 that provides signals Sr+ and Sr− to capacitors C3 and C4 receives the received signals R′x+ and R′x− in the baseband from an amplifier 35, the respective inputs of which are, like the outputs of amplifier 33, connected to a duplexer 36 (4W/2W), the function of which is to perform a 4 wire-2 wire conversion. Circuit 36 generally includes echo cancellation means for eliminating, from the signal received from the line, the echo of the transmitted signal to enable a good reception. The telephone line has been symbolized by its two conductors TIP and RING at the output of duplexer 36.
The operation of an interface system such as illustrated in FIGS. 1 to 3 is known and will not be explained in detail. Only the elements to which the present invention applies, that is, more specifically, the clock transmission through isolation barrier 1, will be reviewed.
FIGS. 4A, 4B, 4C, 4D, and 4E schematically illustrate, in the form of timing diagrams, the clock transmission problem that the present invention aims at solving. FIG. 4A shows an example of a baseband signal meant to cross isolation barrier 1. For simplification, no account will be taken of the differential structure of the signals and only one useful signal has been shown in FIG. 4A. It may be any of signals Tx+, Tx−, R′x+, R′x−. For example, it is assumed that it is signal Tx+ referenced with respect to the common mode voltage VCM of the equipment.
FIG. 4B shows clock signal CK used for the modulation. The signal has been shown as being referenced with respect to common mode voltage VCM due to the multiplication by 1 and −1 effected by the modulator.
FIG. 4C illustrates the shape of signal St+obtained at the output of modulator 21. This signal includes rectangular pulses at the frequency of clock signal CK in an envelope formed with signal Tx+ and its inverse.
FIG. 4D shows an example of the shape of clock signal CK′ recovered on the side of circuit 3. FIG. 4E shows the shape of signal T′x+ recovered at the output of demodulator 31. The demodulation is performed, like the modulation, by a multiplying by 1 or −1 by means of the clock signal, here by a multiplying of signal St+ by signal CK′.
An example of interface to which the present invention more specifically applies is described in U.S. Pat. No. 5,500,895, the content of which is incorporated in the present description by express reference.
A problem that is raised in the type of interface system has to do with disturbances that may affect clock signal CK′ and that originate from radioelectric disturbances due, for example, to electric household appliances (for example, the starting of a motor or of a compressor of a refrigerator).
In conventional systems, such disturbances cause a phase inversion of the clock restored on the line side. Now, when clock signal CK′ correctly restores signal CK, the shape of signal Tx+ is recovered. However, if the phase of signal CK′ is inverted with respect to signal CK, for example, due to a parasitic disturbance p (FIG. 4D), the restored signal T′x+ then is in phase opposition with respect to signal Tx+. The modem that notices the error by checking algorithms must then reposition its demodulator on the new phases relation (the involved demodulator is that, not shown, of the actual modem, downstream of the receive head, and not the demodulator associated with the isolation barrier). Now, each disturbance of the modem reception causes a decrease of the transmission level to enable the modem algorithms to correct the received data. Further, once a modem has switched to a lower transmission level, it does not recover by itself to a better level until the end of the communication.
In conventional systems, the initial state of the regeneration circuit most often is random. It is thus possible to be, as soon as the beginning of a communication, in clock phase opposition. In this case, the modem already switches to a first lower level. If, afterwards, during the communication, a new parasitic pulse occurs, the modem switches to row a still lower level, due to the new clock phase inversion.
It should be noted that processing circuit 3, on the line side, performs other functions than those illustrated in FIG. 3. In particular, this circuit is used to detect the presence of a ringing and to detect a standardized line impedance (for example, on the order of 600 ohms). In certain cases, other capacitors are used in the isolation barrier to transmit other types of signals.